// Private functions:
// -------------------------------------------------------------------------
private Polygon addRing(Polygon polygon) {
LinearRing interiorRing = polygon.getGeometryFactory().createLinearRing(new Coordinate[] {});
AddRingOperation op = new AddRingOperation(interiorRing);
index.setInteriorRingIndex(polygon.getNumInteriorRing());
index.setExteriorRing(false);
return (Polygon) op.execute(polygon);
}